"Presidents have an impact on the nature of our nation, and trickle-down racism, trickle-down bigotry and trickle-down misogyny - all these things are extraordinarily dangerous to the heart and character of America," Romney said in Utah.

He said he expected Trump to get the Republican nomination, but added that he would not vote for either Trump or Clinton.

Donald Trump: Politics as media spectacle - The Listening Post

Romney left open the possibility of casting a ballot for the Libertarian Party candidate, Gary Johnson, a former New Mexico governor.

On Tuesday Paul Ryan, the top elected US Republican and Speaker of the House of Representatives, criticised Trump's remarks on the judge for what he called a "textbook definition of a racist comment".

Unlike Romney, Ryan has endorsed Trump as the Republican nominee.

Ohio Governor John Kasich, who was a nominee for Republican candidacy earlier in the year, said on Thursday that he still was not ready to endorse Trump.

"Why would I feel compelled to support someone whose positions I kind of fundamentally disagree with?" he told Fox News.

As the presumptive nominee, Trump now has to balance maintaining the outsider style that helped propel him to the nomination, while courting Republican insiders, who could be critical to financing a general election campaign against a well-funded Clinton.

Calming concerns

On Friday, at a Christian evangelical conference, Trump offered a message of ethnic harmony as he sought to calm concerns about his criticism of the Mexican-American judge.

In a departure from his usual freewheeling style, Trump read a carefully scripted speech from a teleprompter as part of a new push by his campaign to tone down his harsh rhetoric.